In a conventional engine starter (hereinafter referred to as starter), a start operation is carried out while an engine is stopped. Thus, a pinion gear meshes with a ring gear while the ring gear is not rotating. However, in a system for carrying out idle stop for reducing fuel consumption, a restart property is secured by meshing the pinion gear with the ring gear even when the ring gear is rotating.
For example, at the moment when the idle stop is just started and the engine is not stopped yet, if a restart is requested, or if it is necessary to reduce a period for a restart from a stop state, while the ring gear is rotating, the ring gear is meshed in advance with the pinion gear.
In this case, as a method of meshing the pinion gear with the ring gear while the ring gear is rotating, there is known a method of meshing the pinion gear by supplying an electric power to thereby adjust the speed of the starter motor of the pinion gear so that the pinion gear is synchronized with the RPM of the ring gear (for example, refer to Patent Literature 1). Moreover, there is known a method of carrying out, by providing a mechanism for synchronization in advance, synchronization up to a predetermined difference in RPM by friction of a portion of the mechanism, and then meshing gears with each other (for example, refer to Patent Literature 2). Further, there is known a method of facilitating the meshing by devising the pinion shape (for example, refer to Patent Literature 3).